A projection type display apparatus and a rear projection type display apparatus are among the known large-screen display apparatuses. While these apparatuses have used high-pressure mercury lamps as a light source, research has been going to develop a projection type display apparatus that uses laser beams of three primary colors for the reasons of better color reproduction performance and less power consumption. Unlike the light emitted by a high-pressure mercury lamp, a laser beam is a coherent and monochromatic light and therefore has a possibility of getting focused at a point on the retina of a human eye and impairing the retina. For this reason, products using lasers are classified according to the international standard IEC60825 or, in Japan, by JIS C6802: 2005, that stipulates guidelines for each class of products to be observed by manufacturers and users, to ensure the safe use of laser products.
JIS C6802: 2005 defines a term “maximum permissible exposure (MPE)” that specifies the values of intensity, wavelength, laser source size, duration of exposure, human body tissue that may be damaged by the laser beam, pulse width and other parameters that ensure that the laser beam does not harm a human body when exposed to a beam under normal operating environment. The MPE is determined by a working group of the International Electrotechnical Commission (IEC), based on information obtained from cases of accidents involving lasers and experimental studies using animals. A projection type display apparatus of the prior art has been provided with such safety measures as the intrusion of person into a region of picture projection is detected so as to shut down the laser output or decrease the output power thereof to a safe level below the MPE value, to prevent the human retina from being impaired by direct irradiation with the laser beam (for example, refer to Japanese Patent No. 2994469).
The following problems exist with the configuration of the prior art described above.
Firstly, since the laser output power is decreased upon detection of human intrusion into the region of picture projection, emission of laser beam of high output power may remain uninterrupted, if the intrusion detecting system fails or, even if the intrusion detecting system functions, if there is trouble in the unit of the laser shutdown system. Such a failure may allow a person to approach the projection type display apparatus and look into the laser beam resulting in the impairment of the retina.
Secondly, not only projection type display apparatuses, but also any apparatus that employs laser has an interlock to disable laser emission when a casing is opened to make access to the laser source, thereby ensuring safety. However, there remains a possibility of the laser source being removed from the apparatus and used for unintended purpose by an individual with malicious intent.